The saros is a period of 223 Synodic months(approximately 6585.3213 days, or nearly 18 years 11 days), that can be used to predict eclipses of the Sun and Moon. One saros after an eclipse, the Sun, Earth, and Moon return to approximately the same relative geometry, and a nearly identical eclipse will occur, in what is referred to as an eclipse cycle. A sar is one half of a saros.
A series of eclipses that are separated by one saros is called a saros series.
The earliest discovered historical record of the saros is by the Chaldeans (ancient Babylonian astronomers) in the last several centuries BC. It was later known to Hipparchus, Pliny the Elder, Ptolemy, but under different names. The Sumerian/Babylonian word "šár" was one of the ancient Mesopotamian units of measurement and as a number appears to have had a value of 3600. The name "saros" (σάρος) was first given to the eclipse cycle by Edmond Halley in 1691, who took it from the Suda, a Byzantine lexicon of the 11th century. The information in the Suda in turn was derived directly or otherwise from the Chronicle of Eusebius of Caesarea, which quoted Berossus. Although Halley's naming error was pointed out by Guillaume Le Gentil in 1756, the name continues to be used.
Mechanical calculation of the cycle is built into the Antikythera mechanism.
What is a Saros cycle?Edit
An eclipse is a relatively rare coincidence of several factors at once in order to make the shadows line up properly. First of all, it must be a new moon for a solar eclipse or a full moon for a lunar one, so the Sun, Moon and Earth lie in a straight line. Secondly, these 3 bodies must line up "vertically" so the eclipse shadows aren't too far north or south to be seen from Earth. This means the Sun and Moon must be "near" one of the Moon's Nodes, where the paths of the Sun and Moon cross. ("Near" is roughly an 18 degree orb.) Finally, the difference between a total eclipse of the Sun and an annular eclipse is in how far apart the Moon and Earth are. At the "perigee" point, when they are closest together, the Moon's umbral shadow (the dark part of the shadow, where totality occurs) can reach all the way to the Earth's surface and form a total eclipse. At the "apogee" point, on the other hand, when the Moon is farthest from the Earth, the tip of the Moon's umbral shadow dangles thousands of kilometers above the Earth's surface, never touching, and we see only an annular eclipse.
So to predict future eclipses, we need to understand 3 interlocking cycles, namely:
- The period of time from one new or full moon to the next. Astronomers call this the "Synodic Month" and have measured its average length as 29.53059 days.
- The period of time it takes the Sun to travel from the Moon's North (or South) Node around the zodiac and back, called the "Draconic Year". Because the Moon's Nodes move backwards 19-20 degrees a year, the Draconic Year is shorter than the usual calendar year by several weeks. It's average length is 346.62005 days.
- The period of time from perigee to perigee in the Moon's orbit, called the "Anomalistic Month". This period averages 27.55455 days.
By "coincidence", these cycles all repeat nearly exactly every 18+ years. This is because:
- 223 Synodic Months = 6585 days, 7 hours, 43+ minutes,
- 19 Draconic Years = 6585 days, 18 hours, 44+ minutes,
- 239 Anomalistic Months = 6585 days, 12 hours, 53+ minutes.
This period of 6585.322 days is called a "Saros Cycle" (named from a Greek word meaning "to repeat"). This is roughly 18 years, 11 days and 8 hours (depending upon how many leap days there are on the calendar). So if there's an eclipse now, in 6585.322 days from now,
- There will be another new or full moon occuring,
- The Sun and Moon will be roughly the same distance along the zodiac from the same Lunar Node again,
- The Earth and Moon will be roughly the same distance apart as they are now.
In short, all the factors that make an eclipse happen now will repeat (with surprising accuracy!) exactly one Saros cycle from now and another eclipse (of very similar geometry) will happen then. This Saros cycle is the very powerful predictive tool that the Babylonians were so clever to discover.
Please note that the eclipses in a Saros Family do not happen at the same zodiac position each time. It takes 6798 days (about 18.61 years) for the Moon's Nodes to retrograde around the zodiac and return to the same position. In one Saros period, the Nodes are roughly 11-12 degrees short of returning. So from one Saros period to the next, the Node's move eastward (in the same direction as the Sun moves through the zodiac signs) 11-12 degrees. Therefore, all the eclipse points also drift 11-12 degrees each Saros, gradually sweeping around the zodiac over the centuries. This is the "11+ days" part of the Saros period in action (the Sun moves roughly 1 degree per day).
Why do these Saros families evolve over time?Edit
If these 3 cycles repeated exactly every 6585.322 days, eclipses would repeat perfectly every Saros Cycle. However, these cycles are slightly out of sync with each other, causing the geometry of one eclipse to change slightly when its "Saros buddy" comes around 18+ years later. In particular, notice that 19 Draconic Years minus 223 Synodic Months is about 11 hours. This 11 hour gap is the key to explaining why Saros Families evolve over time.
If you have an eclipse A at one time, and eclipse B one Saros period later, the Sun and Moon will line up again to form a new or full moon. However, the Sun will not have returned to its original distance from the Node (the position of eclipse A), since it has 11 hours yet to travel before it completes that 19th Draconic Year. The Sun moves an average of 27 minutes of arc (roughly half a degree) in 11 hours, so eclipse B has drifted half a degree westward relative to the Node compared to A. Remember that the Sun and Moon must be within 18 degrees of the Node for an eclipse to happen. Because of this half-degree drift, Saros families have a beginning and an end, typically encompassing 70-80 consecutive eclipses before the family quits producing. This entire life cycle of a Saros Family lasts 12 to 14 centuries (70 or 80 times 18+ years).
Assume we are looking at a Saros Family of solar eclipses where the Sun is at the North Node. The first eclipse in the family will happen when the Sun is about 18 degrees east of the Node and hence the Moon is about 1.5 degrees north of the ecliptic. The Moon's shadow is so far north that it only grazes the Earth's surface briefly near the Arctic regions, forming a partial eclipse near the North Pole. The next eclipse 18+ years later will be a half-degree closer to the Node, bringing the shadow path farther south. The eclipse will be "deeper" (more of the Sun's disk is covered up), longer in duration, and cover more of the Earth's surface. In addition, the shadow path moves southward towards the equator. Eventually, after 10-11 eclipses, the Moon's umbra touches the Earth, causing a total (or annular) eclipse. The eclipses that follow will all be total (or annular), with the longest duration eclipses happening near the equator halfway through the Saros Family (after 35-40 eclipses, when the eclipse points are right at the Node). The second half of the Family is the mirror image of the first half. The eclipses continue their march towards the south as the eclipse points fall behind the Node. After a number of total eclipses, the Family finishes up with 10-11 partial eclipses around the South Pole. Eventually, the new moon is too far from the North Node, causing the Moon's shadow to completely miss the Earth, and the Saros Family comes to a close.
If the solar eclipses are at the South Node, the cycle is the same except that the eclipses start at the South Pole and march towards the North Pole.
A similar evolution occurs with the Saros Cycle for a family of lunar eclipses. If the Sun is at the North Node, the Moon is at the South Node. At the first eclipse of the Saros Family, the Moon is east of the South Node, hence south of the ecliptic, and just grazes the "lower" (southern) part of the Earth's shadow. Initially, the eclipses are an appulse (where the Moon passes only through the Earth's penumbra, the light part of the shadow). With each eclipse in the Saros Family, the eclipse point drifts towards the Node more, hence closer to the ecliptic and closer to the center of the Earth's shadow. As the eclipses eventually enter the Earth's umbra (the dark central part of the shadow), they become partial and then total lunar eclipses. In the second half of the Saros Family, the eclipse points fall behind the South Node and the Moon moves further north each time. Total eclipses turn to partial ones, then appulses. Finally, the full moon moves too far away from the Node and the Family stops. If the Moon is at the North Node, reverse these directions.
How many Saros families?Edit
As I've already said, eclipses only happen within 18 days of when the Sun is near one of the Moon's Nodes. These month long windows are called "eclipse seasons"; between 1 and 3 eclipses can happen each window. With eclipses happening every half Draconic year (173+ days) and 19 Draconic years to a Saros, it's obvious that not all eclipses are part of the same Saros Family. In fact, each eclipse season is a different Saros family of solar eclipses and similarly for lunar eclipses.
In addition, as old Saros Families die out, new cycles begin to replace them and frequently overlap in time. Hence there can be multiple Saros Families active in a single eclipse season. In general, there will be 38-42 solar Saros Cycles active at any given time and a similar number of lunar Saros Cycles, each at different stages of their evolution. The total pattern of Saros Cycles is very complicated.
Numbered Saros cyclesEdit
Obviously with this much confusion going on, it's important to come up with some way to talk about individual Saros Cycles and tell one from another. This requires that we give these cycles some kind of name or number that identifies it. Astronomers have one scheme for numbering Saros cycles, while astrologers have their own scheme.
The astronomical scheme for numbering Saros Cycles is based on a 1955 paper published by a fellow named van den Bergh. Ignoring some crucial details, he more or less numbers the Saros' consecutively as new ones start up, beginning at some remote time in ancient history. If the Sun is near the North Node, the Saros is given an odd number; if near the South Node instead, the Saros number is even. The Saros Cycles for solar and lunar eclipses are numbered separately (be careful you keep straight whether it's solar or lunar when talking about a Saros number!). Once a Saros Cycle is complete after 12-14 centuries, its number is permanently retired. The Saros numbers for solar eclipses are currently (2002) in the range of 117 through 155, while lunar eclipses have Saros numbers between 109 and 149.
The Saros numbers that astrologers use is more complicated and confusing (in my mind). First of all, I've only seen two astrology books that discuss the Saros and both of them only work with solar eclipses. Lunar eclipses are ignored, a big mistake in my estimation. They arbitrarily number consecutive eclipse seasons "1 North" (with the Sun at the North Node), "1 South" (South Node), "2 North", etc. all the way up to "19 South". In addition, if there are 2 eclipses in the same eclipse season (29 days apart), the older Saros Family is called "old" and the younger one "new". This leads to long names like "9 South Old" for a Saros that started in 727 AD, and the newer "9 South New" that began in 1917 AD. When the old cycle dies off, the younger Saros drops "new" from its name. Since names are reused and occasionally change as Saros Cycles come and go, this second scheme seems like confusion waiting to happen.
In the tables of active Saros cycles, I'll use the astronomer's method of numbering for clarity. The current equivalent in the astrologer's scheme will also be given for solar eclipses.
How do you use the Saros in interpretting an Eclipse?Edit
Most astrologers are used to thinking of individual eclipses as isolated events with no connections to other eclipses (except by aspect). We analyze each eclipse chart by itself and it never occurs to us to put each eclipse energy in a particular context.
However, the Babylonian astrologers found it was very important to study an eclipse in the context of the Saros Family that contains it. The entire cycle has a "personality" all its own that colors each individual chart. This Saros personality is based on the first eclipse in the Saros Family (a birthchart for the entire cycle, so to speak) and each eclipse in the Family is helping to advance this initial chart to the next level. This may help explain why all eclipses are not created equal -- there are about 80 different Families in action, each with a different pedigree.
It would be interesting to look at the chart for the last eclipse in a Saros Cycle as a way to determine the final outcome of the eclipses happening now. In astrology, future events sometimes reach back in time to the present in order to mold events to a future outcome. It's important to know where you're headed, not just where you came from. I suspect other eclipses that mark changes in the eclipse geometry, especially the first and last total eclipse in the cycle, may also shed some interesting light.
Finally, when looking at an upcoming eclipse, check out its Saros buddy from 18+ years ago. Family members in a Saros are connected and carry some of the same energy threads. Checking your "Saros return" every 18 years can provide illuminating information about long term cycles in your life.
You can study eclipses both as transits (the current eclipses and how they affect us) and natally as prenatal eclipses. These are described next.
Eclipses as TransitsEdit
When you are studying how an eclipse will affect you, you set up a chart for the date and time of the middle of the eclipse (which may differ from the time of the new or full moon by a few minutes, curiously) for the location where you live. If you are looking at another place on the earth's surface, use that location instead. For instance, to see "How will the eclipse affect the US?", you would probably want to set up the chart for Washington, DC. You are looking for close connections between the eclipse chart and your own birthchart, such as close aspects from the eclipse to your chart. The more connections, the stronger and more broad-based the influence is for you. Also pay attention to which natal house contains the eclipse degree. The affairs of that house will likely be up for review and overhaul the next few months.
However, set up a second chart for the eclipse that started the Saros series that contains the current eclipse you are studying. I'd advise using where you live as the place for this chart. (Remember that the initial and final eclipses in a Saros series fall near the north or south pole, where almost all house systems fall apart -- it's nearly impossible to draw "the chart" for such an event.) This second chart describes the energies of the entire Saros, of which the current eclipse is just a member. It's the background energy that colors the current situation, much like the actions of the slower moving planets provide a background for the other transits. This background energy of the Saros chart will also make connections to your own birthchart which you can study. In particular, the house containing this initial eclipse point will probably have some deeper connection affecting the house containing the current eclipse.
As a practical matter when studying these eclipse charts, it's best to stick to the "major" aspects, namely conjunction, square and opposition. Trines and sextiles are perhaps important, though many astrologers would ignore them in this context. Also, it's best to use tight orbs for these aspects, perhaps as little as 2 to 5 degrees.
The last solar and lunar eclipses prior to your birth are known as your "prenatal eclipses". They are often considered to have an important formative effect on the person since we tune into their energies while in utero. The new and full moon charts prior to birth would also be important, though to a lesser degree. I would now add the two charts for the initial eclipses of the Saros cycles for these two prenatal eclipses. Use your birthplace to set up these charts.
It's difficult to explain the strange quality of these eclipse charts. When I first studied my own, I was fascinated by the uncanny feeling in me of watching my soul try to enter this life. The prenatal eclipse charts are similar to my birthchart, but with important differences (such as planets being in a different zodiac sign, or moving from retrograde to direct motion). These differences persisted right up to the new moon before my birth, when my chart finally "snapped into place" and took its form as my birthchart. I imagined my soul coming from a far away place beyond the solar system, squeezing into a tiny human body over time, changing from whatever form it takes out there to a body constrained by the physical plane.
When you set up the initial Saros charts for your birthplace as well, a whole new set of messages comes through. Planets that are important in my birthchart are in critical degrees (such as the Ascendent or Midheaven) of these Saros charts. Planets often fall in degrees that are important in my chart or the charts of people I know. It makes you wonder how long your soul has been thinking about this incarnation.
I suspect these charts are describing reincarnational information, if you can figure out how to decode it. I'd also try looking at Astro*Carto*Graphy and Local Space Maps for these various eclipse charts as a way to understand which parts of the earth were important in your development.
Saros Return ChartsEdit
Every 18+ years, you have a pair of "Saros Return" eclipses. These are the eclipses that are Saros buddies for your solar and lunar prenatal eclipses. These Saros return charts are akin to progressing your prenatal eclipse charts into the future. They show how you are evolving as a "carrier" of the energies of the original eclipse charts, and the changing demands as you face the next 18 years. While I haven't studied these charts much, my gut instinct is they should be quite revealing.